1. Field of the Invention
This invention relates to a liquid interface diffusion bonded composition and a method of preparing such a composition. More particularly, this invention relates to a liquid interface diffusion bonded composition comprising at least one metal (e.g. titanium) honeycomb core which is bonded to a metal (e.g. titanium) facing sheet, wherein a foil interlayer comprising zirconium, copper, nickel and titanium is used to join the faying surfaces of the honeycomb core and facing sheet by being rendered liquid at the bonding temperature and thereby forming a liquid interface for effecting diffusion bonding of the core and facing sheet. The composition and method of this invention are useful where high strength, lightweight materials are required, such as in aircraft and aerospace-related structures.
2. Background Information
Brazing and diffusion bonding methods for the joining of honeycomb sandwich panels and other titanium structures are known to those skilled in the art. However, as described for example in U.S. Pat. Nos. 3,768 985 and 3,769,101, such brazing and diffusion bonding techniques have not been found entirely satisfactory, especially for joining titanium-based components such as joining a titanium honeycomb core material to a titanium facing sheet. In response to the problems associated with such brazing and diffusion methods, U.S. Pat. No. 3,768,985 describes a combined brazing and diffusion process referred to as a liquid interface diffusion (LID) process is described for joining a titanium honeycomb core and a titanium facing sheet bonded thereto. A brazing or filler material comprising 38% Cu, 38% Ni and a balance of Ag (by weight) is interposed between the faying surfaces of the honeycomb core and facing sheet, and the brazing material is rendered liquid at the brazing temperature to form a liquid interface between the faying surfaces which establishes the required metal-to-metal contact therebetween so that atomic transport can be effected and diffusion accelerated. In addition, U.S. Pat. No. 3,769,101 describes a LID process in which a small amount of three or more selected metals are interposed between the faying edges of the material to be bonded, to form a diffusion bridge. The selected metal may be Cu, Ni and Ag, or Cu, Ni and one low melting point metal selected from the group consisting of Sb, Bi, Cd, Sn, Zn, Au and Pt.
The use of amorphous Ti-based brazing alloys for diffusion bonding of thin sheet structures of titanium and its alloys is described in B.A. Kalin et al., xe2x80x9cBrazing Thin Sheet Structures of Titanium Alloys Using CTEMET Amorphous Brazing Alloys,xe2x80x9d in Welding International, pp. 234-35, (1997). However, relatively short bonding times (i.e. 5-30 minutes) are described, which tend to limit the degree of atomic diffusion and homogenization of the joint formed between the honeycomb core and facing sheet.
Although the above-described LID processes are useful in overcoming the problems associated with traditional brazing and diffusion bonding methods, there are several additional problems associated with the LID process itself. For example, it is often difficult to achieve sufficient bonding of mismatched faying surfaces. In addition, disbonding of the surfaces after initial bonding also may occur, which may require post-processing repairs such as the introduction of pins and the like to join the surfaces with sufficient mechanical integrity. In view of the foregoing, it would be desirable to employ a LID process which is capable of sufficiently bonding slightly mismatched faying surfaces, and reduces the incidence of disbonding of the initially bonded surfaces and the concomitant necessity to use pins and the like to sufficiently join the surfaces.
This invention uses a relatively thick (0.0008-0.0012, preferably about 0.001 in.) foil interlayer of an alloy where Cu and Ni are diluted with Ti and Zr. This allows for a thicker braze foil interlayer than would be possible with an alloy of just Cu and Ni, particularly in applications involving the joining of thin gage titanium honeycomb materials. The thick foil interlayer provides advantages of decreasing the incidence of disbonds in slightly mismatched faying surfaces. The diluted composition also aids the dilution process during bonding. Moreover, the longer bonding
times (i.e. at least 3 hours) used to prepare the composition of this invention enhances the degree of atomic diffusion and homogenization of the joint formed between the honeycomb core and facing sheet.
It is one object of this invention to provide a liquid interface diffusion bonded composition in which at least one metal honeycomb core is bonded to a metal facing sheet, wherein a foil interlayer comprising zirconium, copper, nickel and titanium is used to join the faying surfaces of the honeycomb core and facing sheet by being rendered liquid at the bonding temperature and thereby forming a liquid interface for effecting diffusion bonding of the core and facing sheet. It is another object of this invention to provide a method of preparing such a composition. The composition of this invention advantageously enables the bonding of such metal components, including the bonding of slightly mismatched faying surfaces of such components , and reduces the incidence of disbonding of the initially bonded surfaces and the concomitant necessity to use pins and the like to sufficiently join the surfaces. The composition and method of this invention are useful in applications where high strength, lightweight materials are required, such as in aircraft and aerospace-related structures.
The liquid interface diffusion bonded composition of this invention comprises a metal honeycomb core and a metal facing sheet bonded thereto, wherein the composition is prepared by a method comprising:
(a) providing a metal honeycomb core having a faying surface and a metal facing sheet having a faying surface;
(b) placing together the honeycomb core faying surface and the facing sheet faying surface, and providing therebetween a metal foil comprising about 10.5-12.5 wt. % zirconium, about 20-24 wt. % copper, about 10.5-16 wt. % nickel, and the balance being titanium;
(c) subjecting the faying surfaces and metal foil therebetween to sufficient positive pressure to maintain position and alignment for joining; and
(d) heating the faying surfaces and metal foil therebetween in a protective atmosphere to a temperature in the range of 1700-1800 degrees F. for at least 3 hours to cause a melt of the metal foil.
In particularly preferred embodiments of the composition and method of this invention, the honeycomb core is a titanium honeycomb core, the facing sheet is a titanium facing sheet, and the metal foil is formed by a rapid solidification process or a melt spinning process. The composition and method of this invention are useful in applications where high strength, lightweight materials are required, such as in aircraft and aerospace-related structures.